1. Field of the Invention
This invention is related to a device, and a method of using the device, that is used to measure various photometric quantities of the light emanating from a light source. Generally a light source, particularly an exterior light source, is configured and positioned to cast light in a desirable pattern. Often the light source will indiscriminately emit light such that the some of the emitted light is directed beyond the intended lighted zone. Light extending beyond the targeted object of the lighting system is “spillover.” When spillover extends beyond property lines it is known as “trespass light” and may be objectionable to people subjected to this intrusive trespass light.
The device presented in this disclosure measures the light from a distant light source. The distant light source may be the source of spillover light, trespass light and a source of glare. The measurement of spillover and trespass light is the target of the measurement equipment presented herein. The measurement of the trespass light will be used to determine the acceptability of the light source configuration, that is, whether or not the trespass light or glare is acceptable or unacceptable in a particular situation.
A telescopic light-sensing meter used to measure illumination levels of distance sources of light, sometimes referred to as a telephotometer, is disclosed herein. The light sensing device, including the telephotometer is connected to a processing unit that will process information from the telephotometer. The processor will also receive information from a range finder. Data from the range finder and the enhanced telephotometer is processed and a finite number is displayed to the equipment user.
2. Description of the State of the Art
Outdoor lighting meters are available to measure illuminance values in lux or footcandles. These state of the art light meters measure only the illuminance of the area on which the light falls. These are simple portable meters with a light sensitive cell that rests on the surface of interest and generates a signal to an appropriate display, which then indicates the illuminance level of the surface being measured. The meter is effective in measuring illuminance on vertical surfaces, such as a vertical window of a residence. The meter may also be used to measure illuminance on a horizontal surface, such as on a floor of a building. This method does not meet today's need of measuring spillover or trespass lighting in terms of light intensity.
Other meters, referred to above and hereinafter referred to as telephotometers are available. Telephotometers use a lens to focus the image of a light source of interest onto a light sensitive cell. Frequently the focusing optics are similar to those used in a telescope and can be used to capture light from a distant source. The quantity measured by a telephotometer is the luminance of the area at which the telephotometer is aimed, luminance being the intensity of light per unit area of the area being measured. Luminance measurements are usually stated as footlamberts or candelas per square meter.
It has been recently realized that there is a need to measure light intensity “in the field” rather then simply in a laboratory setting. There is no known apparatus or method of doing this and thus the inventors herein embarked on the development of a light meter that can measure light intensity in the field and fulfill a heretofore unfilled need in today's lighting intensive society.
In many situations a homeowner or a municipal body may have a complaint of spillover lighting. Unfortunately, up to now, there is no way to measure the spillover or trespass light without placing a meter directly on an affected surface to measure illuminance on the surface. However placement of a light meter close to and proximate a surface is sometimes impractical or impossible. It would be very desirable if there were equipment available to measure the intensity of a particular light source and not just illuminance on a surface lit by the light source. The apparatus presented here fulfills the heretofore unattained goal of field measurement of light intensity by providing a light meter that can measure the intensity of light from a distant light source.
It has also been found that it is more useful to measure intensity of light at a point distant from the light source, rather than to measure the illuminance of light falling on a surface.